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18 virtual memory By Swera Baloch

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Computer Organization • Virtual Memory Virtual Memory  In early computers memories were small and expensive  Programmers of those days spent a lot of time squeezing programs into tiny memories.  Often it was necessary to use algorithms that (though slower) used less space as programs written with the faster algorithms were too big.  One early solution to the problem was the use of overlays The programmer would break a program into several sections. The first overlay would run, complete its task then load the next portion The second portion would “overlay” the first portion, run, then if needed call another overlay (even the one that previously called it). Virtual Memory • In 1961 a group of researchers at Manchester, England proposed a method for performing the overlaying process automatically without the programmer knowing • This method called “virtual memory” was first used on research computers during the 1960’s • The address space viewed by the program was seen as being broken up into smaller chunks which are automatically moved in and out of memory as the program runs. Virtual Memory  There are two distinct virtual memory techniques: Paging Fixed sized portions of a program are swapped in and out of memory as needed Segmentation Program is broken into variable size logical units (e.g. instructions, data, etc.) Theses parts are swapped in and out of memory as needed  Of course, there is also segm ...
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